The prior art discloses numerous paint delivery and application systems, including systems using pigging technology. Such systems may, for example, be used to apply seriatim, paints of different color to a plurality of substrates, such as vehicle bodies on a moving conveyor in a paint spray booth. The term “paint,” as used herein includes solvent or water base paints used to paint or coat a substrate and generically any coating, including protective coatings, which may be applied to a substrate using an applicator, such as a sprayer or rotary atomizer. Depending on its function in the coating delivery system, pigs or pigging elements are variously referred to in the prior art as pigs, shuttles, separating elements, terminating pistons, plugs, etc. For the purposes of this application, the term pigging element is used. Generally, a pigging element is utilized in prior art paint delivery systems to push paint either toward the applicator to apply the paint, separate different paints or solvents, separate or clean the paint supply lines or push paint in the supply lines back to the paint switching device or color changer. Pigging elements have also been used to separate fluids in a delivery line including paints of different colors, solvents, viscous liquids, oils, etc. which may be driven through the delivery line by pneumatic pressure or other fluid.
A conventional paint supply and application system includes a source of paint under pressure, an applicator, such as a rotary atomizer or sprayer, generally at high voltage where the paint is conductive and applied with an electrostatic rotary atomizer, a source of solvent under pressure and a supply line or lines connecting the source of paint and solvent to the applicator. Where the source of paint and solvent includes a color changer or paint supply switching device, the paint supply and application system may also include a paint canister which may be mounted on a robot, for example, and the color changer is generally at ground potential. When a second paint is to be applied, the canister may be switched for a canister filled with a different paint or the canister may be connected to a source of different paint. As used herein, the term “canister” or “paint canister” includes any container suitable for receipt and delivery of paint.
A paint delivery and application system or apparatus using pigging technology for application of different paints, as disclosed in the prior art, typically includes a color changer or paint supply switching device, including sources of different paints and a source of solvent under pressure, a paint applicator, such as a rotary atomizer, a supply line connected to the color changer and the applicator and a pigging element which is received in the supply line to push paint through the supply line either toward the paint applicator to apply paint to a substrate or from adjacent the paint applicator to the color changer to recover paint in the supply line. As the pigging element moves through the supply line, it pushes paint or solvent through the supply line and cleans or scrapes paint or solvent from the supply line. In one embodiment disclosed in the prior art, for example, a pigging element is inserted in the supply line to push paint from a color changer to the applicator and the pigging element is then removed. In another embodiment disclosed in the prior art, the supply line between the paint supply switching device and the applicator includes two pig receiver stations, including a first pig station adjacent the paint supply switching device or color changer and a second pig station adjacent the paint applicator. The pig receiver stations are defined by a chamber which permits paint or solvent to flow around the pigging element. The pigging element is releasably retained in the first pig station by a clamping means and then released to push paint from the first pig receiver station adjacent the paint applicator to push paint in the supply line to the paint applicator. The pigging element is then driven back under pneumatic pressure to the first receiving station to push paint in the supply line back to the paint supply and switching device or color changer to recover paint in the supply line.
Co-pending PCT Application WO 03/14471 filed May 6, 2003 discloses significant improvements in paint delivery and application systems and methods, wherein a preferred embodiment includes a conventional color changer or paint supply switching device at ground potential preferably having at least two sources of paint under pressure, at least two paint canisters, a paint applicator, a first pair of separate supply lines connecting the color changer and the paint canisters and a second pair of separate supply lines connecting the canisters to the paint applicator. Thus, with the disclosed embodiment of the paint delivery and application system, a first paint may be supplied from a first paint canister to the paint applicator while a second paint is supplied from the color changer to a second canister, significantly reducing the cycle time and electrostatically isolating the paint applicator from the color changer or other source of paint. Further, where this system includes pigging technology, a preferred embodiment includes a pig station adjacent each of the paint canisters, which provides a source of paint under pressure, and a second pig station adjacent the paint applicator. In one preferred embodiment, two pigging elements are located in the second supply lines between the paint canisters and the paint applicator and solvent may be received between the pigging elements, such that paint under pressure is directed against the pigging elements from the paint canisters, driving the pigging elements from the first pig station to the second pig station where solvent is located between the pigging elements, the system results in cleaning or purging the supply line and the paint applicator and then directing paint under pressure to the paint applicator, which applies paint to a substrate.
In a preferred embodiment, the pigging elements include annular generally frustoconical skirt portions adjacent the opposed ends having a diameter greater than the supply lines to scrape the supply lines of residue paint from the prior application. However, it has been found that the pigging elements in such applications “chatter” in the delivery lines, skipping and jumping through the delivery lines, which is a particular problem with paint delivery and application systems requiring precise metering of the paint delivered to the applicator, although it is also a problem with other coating applicators requiring accurate metering of the coating. Another problem with such delivery and application systems is wear of the pigging elements. In a preferred embodiment, the pigging elements also include a sensor element, such as a magnet, to permit precise determination of the location of the pigging element in the supply line. Thus, the pigging elements used in such delivery systems are relatively expensive and the pigging elements must be discarded, if worn or damaged.
As will be understood by those skilled in this art, the delivery lines between the source of paint under pressure and the paint applicator may be 14 to 16 meters in length or greater and the pigging elements must be driven from the source of paint to the paint applicators through the delivery lines and typically returned from the paint applicator to the source of paint with each application of paint. In a typical application, the delivery lines are flexible. Thus, the pigging elements must be rapidly delivered through the delivery lines with each application of paint and skipping or jumping of the pigs through the delivery lines is a particular, but not exclusive problem with paint delivery and application apparatus. The paint delivery and application apparatus and method of this invention eliminates the problem of skipping or jumping of the pigging elements through the delivery lines from the source of paint to the paint applicator, resulting in more accurate metering of the paint, reducing wear of the pigging elements and extending the life of the pigging elements in such applications.